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The standby server must either delay application of the WAL records (and everything after them, too) or else cancel the conflicting query so that the DROP TABLE can be applied. The WAL change records come through to the standby while the standby query is still running, causing a conflict. But when DROP TABLE is run on the primary, the primary doesn't have information about what queries are running on the standby, so it will not wait for any such standby queries. If this situation occurred on the primary, the DROP TABLE would wait until the other query had finished. Clearly the standby query cannot continue if the DROP TABLE is applied on the standby. Therefore, a mechanism is provided to forcibly cancel standby queries that conflict with to-be-applied WAL records.Īn example of the problem situation is an administrator on the primary server running DROP TABLE on a table that is currently being queried on the standby server. Furthermore, allowing WAL application to wait indefinitely may be very undesirable, because the standby's state will become increasingly far behind the primary's. However, on the standby there is no choice: the WAL-logged action already occurred on the primary so the standby must not fail to apply it. On the primary server, these cases simply result in waiting and the user might choose to cancel either of the conflicting actions. These can be used to monitor the progress of recovery, or to allow you to write complex programs that restore the database to particular states.Īccess Exclusive locks taken on the primary server, including both explicit LOCK commands and various DDL actions, conflict with table accesses in standby queries.ĭropping a tablespace on the primary conflicts with standby queries using that tablespace for temporary work files.ĭropping a database on the primary conflicts with sessions connected to that database on the standby.Īpplication of a vacuum cleanup record from WAL conflicts with standby transactions whose snapshots can still “ see” any of the rows to be removed.Īpplication of a vacuum cleanup record from WAL conflicts with queries accessing the target page on the standby, whether or not the data to be removed is visible. These allow you to write programs that are aware of the current state of the database. (In server versions before 14, the in_hot_standby parameter did not exist a workable substitute method for older servers is SHOW transaction_read_only.) In addition, a set of functions ( Table 9.88) allow users to access information about the standby server. Users can determine whether hot standby is currently active for their session by issuing SHOW in_hot_standby. Once hot standby finishes, it will be possible to initiate read-write transactions (even from a session begun during hot standby). Sessions will remain connected while the server changes mode. If failover or switchover occurs, the database will switch to normal processing mode. But as long as no attempt is made to modify the database, connections during hot standby will act much like any other database connection. It is possible that some of these restrictions might be loosened in a future release.ĭuring hot standby, the parameter transaction_read_only is always true and may not be changed. In normal operation, “ read-only” transactions are allowed to use LISTEN and NOTIFY, so Hot Standby sessions operate under slightly tighter restrictions than ordinary read-only sessions.
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Two-phase commit commands: PREPARE TRANSACTION, COMMIT PREPARED, ROLLBACK PREPARED because even read-only transactions need to write WAL in the prepare phase (the first phase of two phase commit). SET TRANSACTION READ WRITE, SET SESSION CHARACTERISTICS AS TRANSACTION READ WRITE Transactions started during hot standby may issue the following commands:īEGIN READ WRITE, START TRANSACTION READ WRITE Snapshots may be taken at the start of each query or at the start of each transaction, depending on the current transaction isolation level. Once the commit record for a transaction is replayed on the standby, the changes made by that transaction will be visible to any new snapshots taken on the standby. We say that data on the standby is eventually consistent with the primary. Running the same query nearly simultaneously on both primary and standby might therefore return differing results. The data on the standby takes some time to arrive from the primary server so there will be a measurable delay between primary and standby. All such connections are strictly read-only not even temporary tables may be written. When the hot_standby parameter is set to true on a standby server, it will begin accepting connections once the recovery has brought the system to a consistent state.